<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
</head>

<body>
  <style>
    .parent {
      width: 100%;
      height: 37.5rem;
      background: #09c;
      
    }

    .children {
      width: 100px;
      height: 100px;
      background-color: #090;
    }


    /* 1 position maring */
    /* .parent {
      position: relative;
    }
    .children {
      position: absolute;
      top: 0;
      right: 0;
      bottom: 0;
      left: 0;
      margin: auto;
    } */

    /* 2 */
    /* .parent {
      position: relative;
    }
    .children {
      position: absolute;
      top: 50%;
      left: 50%;
      transform: translate(-50%, -50%);
    } */

    /* 3 */
    /* .parent {
      position: relative;
    }
    .children {
      position: absolute;
      top: 50%;
      left: 50%;
      margin-left: -50px;
      margin-top: -50px;
    } */

    /* 4 flex*/
    .parent {
      display: flex;
      justify-content: center;
      align-items: center;
    }

  </style>

  <div class="parent">
    <div class="children">
    </div>
  </div>
</body>

</html>